1. Field of Invention
The present invention relates to a semiconductor device having a circuit including a thin film transistor (hereinafter referred to as a TFT), and to a method for manufacturing the same. For example, the invention relates to an electronics device on which an electrooptical device typified by a liquid crystal display panel or a light emitting display device having an organic light emitting element is mounted as its component.
In this specification, the term semiconductor device refers to a device in general that utilizes semiconductor characteristics to function, and electro-optical devices, semiconductor circuits and electronics devices are all included in the semiconductor device.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (a TFT) by using a semiconductor thin film (thickness is about from several nm to several hundreds nm) formed over a substrate having an insulating surface has drawn attention. The thin film transistor has gained a wide application in electronics devices such as an IC and an electrooptical device, and particularly, development of a TFT as a switching element for an image display device has been hurriedly carried out
In order to obtain a high quality image in an image display device, An active matrix liquid crystal display device and an active matrix light emitting device as image display devices in which pixel electrodes are arranged in matrix and TFTs as switching elements connected to the respective pixel electrodes are used have drawn an attention.
Such an active matrix display device has been expanded in application. Therefore, the device is required to have high resolution, high opening ratio, and high reliability with the increase in a screen size.
It is possible to form a pixel portion that performs image display, and a driver circuit for controlling a pixel portion such as a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit, or the like based on a CMOS circuit in each functional block, over a single substrate for a liquid crystal module mounted on a liquid crystal display device.
Further, in the pixel portion of the liquid crystal module, a TFT (a pixel TFT) is arranged in each of several tens to several millions of pixels and each pixel TFT is provided with a pixel electrode. A counter electrode is provided on a counter substrate side so that a liquid crystal is interposed between the two electrodes, to thereby form a kind of capacitor with the liquid crystal used as a dielectric. A voltage applied to each pixel is then controlled by a switching function of the TFT to control the application of charge to the capacitor. Thus, the liquid crystal is driven, thereby displaying an image through the control of an amount of transmitted light.
In a liquid crystal display device, when a pixel electrode is formed over an interlayer insulating film having unevenness, unevenness is also formed on the surface of the pixel electrode along the unevenness of the interlayer insulating film. The uneven portion has a possibility to cause an orientation defect of a liquid crystal.
In recent years, research related to a light emitting device having an EL element as a self-luminous light emitting element has been activated. The light emitting device is also referred to as an organic EL display or an organic light emitting diode. Since these light emitting devices have characteristics such as rapid speed of response that is suitable for movie display, low voltage, low power consumption driving, they attracts an attention for a next generation display including a new generation's cellular phone and a personal digital assistance (PDA).
As for the EL element, it is said that by sandwiching an organic compound layer between a pair of electrodes and applying voltage therebetween, an electron injected from a cathode and a hole injected from an anode are recombined at luminescence center in the organic compound layer to form a molecular exciton and when the molecular exciton returns to the ground state, energy is discharged to emit light. There are known singlet excitation and triplet excitation in an excited state, and it seems that light can be emitted by way of either of the excited states.
There are an inorganic light emitting material and an organic light emitting material as a light emitting material used for an EL element, and the organic light emitting material driven with low voltage draws attention.
However, also in a light emitting device using an EL element, when an anode (or a cathode) is formed over an interlayer insulating film having unevenness, there is a possibility that short is generated between the anode and the cathode since a layer containing an organic compound is thin.
Further, an organic EL element using an organic material for an EL element has a problem that luminescence properties such as luminance or evenness of luminescence significantly deteriorate compared to an early period. The low reliability is a factor causing the practical application to be limited.
As one factor of worsening reliability, moisture or oxygen which intrudes into the organic EL element from outside is given.
In an EL display device (panel) using an EL element, moisture intruding inside cause a severe reliability lowering to generate a dark spot, shrink or luminance deterioration from the periphery of a light emitting display device. The dark spot is a phenomenon in which luminance deteriorates partly (including a pixel which does not emit light). The dark spot is generated when a hole emerges on an upper electrode, for example. Shrink is a phenomenon in which luminance deteriorates from an edge of a pixel.
Accordingly, a display device having a structure for preventing the above-mentioned deterioration of the EL element has been developed. In order to prevent the foregoing problems, there is a method in which the EL element is stored in an airtight container to shield from outside air by sealing the EL element in the enclosed space, and a desiccant is further provided separate from the EL element in the enclosed space (for example, Reference 1: Japanese Patent Laid-Open No. Hei 9-148066).
Further, there is another method in which a sealant is formed over an insulator with the EL element formed thereon, and an enclosed space surrounded by a covering member and the sealant is filled with a filler made of a resin or the like; therefore, the EL element is sealed from outside (for example, Reference 2: Japanese Patent Laid-Open No. Hei 13-203076).
In the above-mentioned references, a sealant is formed over an insulator with an EL element formed thereon, and an enclosed space surrounded by a covering member and the sealant is formed.
Briefly, moisture which causes deterioration such as a dark spot intrudes into a display device after sealing in most cases. In addition, moisture or oxygen intrudes mainly through the sealant since an insulator and a covering material are made of metal or glass in most cases.
When the sealant for sealing is on laminated films, all the laminated films are to directly contact with atmosphere outside of a panel. Therefore, moisture or oxygen outside the panel intrudes inside the display device through the laminated films. Further, when a material high in moisture permeability and water-absorbing property such as acrylic is used as an interlayer film, intruding moisture or oxygen is increased.
In addition, in the case of performing patterning of a partition wall, or an aperture step or a washing step of a contact hole after forming the interlayer film, when the interlayer film has a water absorption property, moisture is included inside the interlayer film during these steps.
Moisture or oxygen intrudes from an exterior surface of the interlayer film, then, intrudes into the interlayer film which is directly contact with the EL element after going through a disconnection part and the like generated by poorness of deposition property of source and drain electrodes in the contact hole. Alternatively, moisture included in the interlayer film intrudes into of the EL element. It is thought that various deterioration such as internal contamination of an EL display device, deterioration of electric property, a dark spot or shrink.